A college math professor and his wife are both 60 years old. One evening the wife comes home and finds a note from her husband that says, "My dear, now that you are 60 years old, there are some things you no longer do for me. I am at the Holiday Inn with my 20-year-old student. Don't bother waiting up for me."

He returns home late that night to find a note from his wife: "You, my dear, are also 60 years old and there are also things I need that you're not giving me. So I am at the Motel 6 with one of your 20-year-old students. Being a math professor, I'm sure you know that 20 goes into 60 way more than 60 goes into 20. So, don't you wait up for me."